The present invention generally relates to control of downhole well tools and, in a preferred embodiment thereof, more particularly relates to an electromagnetic telemetry actuated firing system for a well perforating gun.
In a typical construction of a subterranean well, a metal-cased wellbore is extended downwardly through the earth and through a fluid-bearing formation beneath the earth""s surface. To operatively communicate the formation with the interior of the casing for subsequent delivery of formation fluid to the surface, perforations are formed through the casing and outwardly into the formation using a perforating gun structure which is lowered through the casing, typically on a tubing string, to the level of the subterranean formation.
A firing head portion of the lowered perforating gun structure is subsequently actuated to fire the gun and create the desired casing perforations. Perforating gun firing heads are customarily of either a mechanically actuatable or electrically actuatable construction. A mechanical firing head is typically actuated by pressure, or a mechanical device dropped down the tubing to depress a plunger portion of the firing head and thereby initiate firing of the gun. An electrical firing head is typically actuated by an electrical current supplied to a blasting cap attached to the head to detonate the gun charges. Evolving wellbore technologies and completion techniques have surpassed the ability of current tubing conveyed perforating firing systems to fire their guns by the use of pressure or mechanical means. Moreover, due of such evolving wellbore technologies, a variety of wells simply cannot be perforated using conventional techniques.
For the foregoing reasons it can readily be seen that a need exists for improved apparatus and methods for firing perforating guns that eliminate or at least substantially reduce the above-noted problems, limitations and disadvantages typically associated with conventional perforating gun firing apparatus and methods.
In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, a specially designed well tool assembly is provided for operative placement in a subterranean wellbore, the well tool assembly representatively being a remotely actuatable mechanical perforating gun assembly operable to form perforations in a metal casing portion of the wellbore.
The perforating gun assembly, when disposed downhole, is selectively operable by an electromagnetic telemetry actuated firing system that includes a surface-disposed transmitter operable to propagate electromagnetic waves through a portion of the earth exteriorly adjacent the wellbore casing. Preferably, the electromagnetic waves are modulated square sine or cosine waves having a frequency in the range of from about 15 HZ or less, and have a predetermined firing address encoded therein.
The perforating assembly illustratively includes a perforating gun having a mechanically actuatable firing head, an actuating section connected to the firing head and having a motor portion operable to mechanically actuate the firing head, and a receiver operable to detect the electromagnetic waves and responsively operate the motor. The perforating gun assembly may also have a sensor portion for sensing a selected downhole parameter, and a transmitter for propagating through the earth electromagnetic waves indicative of the value of the sensed downhole parameter. These waves may be detected by a suitable surface-disposed receiver.
While the well tool assembly is representatively a perforating gun assembly, other types of well tool assemblies may be utilized if desired and actuated using the electromagnetic telemetry actuating system of the present invention.
According to one aspect of the invention, the tool assembly receiver has a control circuitry portion, and the tool assembly has first and second electrically conductive paths which are insulatively isolated from one another and are respectively operative to transmit an electromagnetic wave signal from a first casing portion to the receiver control circuitry portion with respect to a ground reference from a second casing portion, spaced apart a substantial distance in a downhole portion from the first casing portion, to the control circuitry portion. The receiver control circuitry portion representatively has programmed therein a wave frequency value and a firing address which must be matched with the frequency and firing address of the detected electromagnetic before the circuitry is operative to fire the perforating gun.
Illustratively, the well tool assembly has an elongated, electrically conductive tubular outer body portion and a generally coaxially extending electrically conductive tubular inner body portion, each of the outer and inner body portions having insulative gaps formed therein between adjacent longitudinal sections thereof. Preferably, the adjacent longitudinal sections of the tubular outer body portion has axially spaced apart threaded end portions threadedly connected to an annular collar member at thread joints containing an electrically insulative material defining spaced apart insulation gaps between the longitudinal sections of the outer body portions and electrically isolating them from one another.
According to another feature of the invention, the receiver has a circuit board portion with a main CPU portion adapted to receive an electromagnetic wave detection signal and a ground reference and responsively generate an actuation request signal, and an auxiliary fail-safe CPU portion operative to receive the actuation request signal, monitor selected parameters of the well tool assembly to detect whether system errors exist, and responsively generate a final actuation signal, to actuate the tool portion of the assembly, only in the absence of sensed system errors.
The perforating gun assembly may be operatively supported in the wellbore on a variety of support structures including a tubing string, coil tubing, wire line, slick line or a casing hanger. The electromagnetic telemetry actuated firing system of the present invention provides a variety of advantages over conventional perforating gun firing systems. For example, the system is essentially wireless, with no downhole cabling required.
The motor section of the well tool may have an output member which is translatable in a selectively variable direction through a selectively adjustable stroke. Additionally, the overall well tool assembly may comprise a plurality of separately actuatable well tools which may be actuated in any desired sequence.